Thelazia spp

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Conjunctivits in a dog with Thelazia callipaeda[1]
High parasitic burden in a positive dog with conjunctivitis and oedema.[2]

Thelazia spp (eye worm) are a rare nematode parasite of dogs and cats, transmitted by fruit flies.

First identified in Thailand more than 100 years ago, they have been regularly reported in South-East Asian countries in predominantly low socioeconomic regions.

Adult female Thelazia deposit embryonated eggs in the ocular tears of dogs and are ingested by the fruit fly (Phortica variegata)[3], wherein they develop into larvae in the fly’s body cavity over a period of 2 - 4 weeks.

Once developed to infective larvae (L3), they migrate via the internal organs into the fly’s proboscid, where they are deposited onto the face of dogs while feeding. L3 larvae then migrate to the conjunctival sac, where they fed off bacteria within ocular secretions.

Autochthonous (self-reinfecting) cases in both dogs and cats in south-western France[4] and Switzerland have been reported[5].

Species which are pathogenic to dogs include:

  • Thelazia callipaeda[6]
  • Thelazia californiensis[7] (Western USA only)
  • Thelazia gulosa
  • Thelazia lachrymalis
  • Thelazia rhodesi
  • Thelazia skrjabini

Both the larvae and adults are involved in the pathogenesis of ocular disease. Clinically infected dogs present with mild exudative conjunctivitis, blepharitis, epiphora, periocular pruritus and in severe cases, corneal edema and keratitis, due to the mechanical trauma of the larvae[8]. Blindness may eventually result if left untreated[9].

Ophthalmological examination usually reveals bulbar and nictitating membrane conjunctival hyperemia with serous discharge noted at the left medial canthus[10].

Diagnosis is based on presenting clinical signs and identification of eggs or adult worms in ocular secretions. PCR assays are also available for speciation of the parasite.

Treatment is relatively effective with mechanical removal of worms and administration of oral or subcutaneous doramectin or ivermectin.

Additional topical application of ophthalmic fusidic acid drops and oral milbemycin oxime may assist rapid resolution of clinical signs.


  1. Motta B et al (2012) Therapeutic efficacy of milbemycin oxime/praziquantel oral formulation (Milbemax®) against Thelazia callipaeda in naturally infested dogs and cats. Parasit Vectors 5:85
  2. Miró G et al (2011) Thelazia callipaeda: infection in dogs: a new parasite for Spain. Parasit Vectors 4:148
  3. Otranto D et al (2006) The zoophilic fruitfly Phortica variegata: morphology, ecology and biological niche. Med Vet Entomol 20:358–364
  4. Dorchies P et al (2007) Reports of autochthonous eyeworm infection by Thelazia callipaeda (Spirurida, Thelaziidae) in dogs and cat from France. Vet Parasitol 149:294–297
  5. Malacrida F et al (2008) Emergence of canine ocular thelaziosis caused by Thelazia callipaeda in southern Switzerland. Vet Parasitol 157:321–327
  6. Bhaibulaya M, Prasertsilpa S, Vajrasthira S (1970) Thelazia callipaeda Railliet and Henry, 1910, in man and dog in Thailand. Am J Tropical Med and Hygiene 1970:476-479
  7. Kirschener BI, Dunn JP, Ostler HB (1990) Conjunctivitis caused by Thelazia californiensis. Am J Ophthal 1990:573-574
  8. Otrantoa, D & Traversa, D (2005) Thelazia eyeworm: an original endo- and ecto-parasitic nematode. Trends in Parasitology 21(1):1-4
  9. Anderson RC (2000) Nematode parasites of vertebrates: their development and transmission. CABI Publishing, Guilford, UK. pp:404–407
  10. Pimenta P et al (2012) Canine ocular thelaziosis caused by Thelazia callipaeda in Portugal. Vet Ophthalmol Oct 15